Purification and characterization of constituent androstenedione 15 alpha-hydroxylase (cytochrome P450(15 alpha AD)) from mouse liver. Sex- and tissue-dependent expression

Spurious observation of splenic cyp2b1 expression

Phenobarbital (PB) induction of the CYP2B subfamily was studied in the livers and spleens of male and female rats. Animals were treated with either PB (10 mg/kg) or vehicle for 4 consecutive days. A reverse transcriptase-polymerase chain reaction (RT-PCR), quantitative Northern blotting, Western blotting, and a radioenzymatic assay were used to observe differential levels of CYP2B1 and CYP2B2 mRNAs, proteins, and catalytic activities. CYP2B2 expression was limited to the livers of PB-treated male and female rats and was not detected in spleen. Low constitutive levels of CYP2B1 mRNA were markedly induced approximately 7- to 17-fold in the livers of PB-treated male and female rats, respectively. However, using the same standard oligonucleotide probe for CYP2B1 mRNA, we observed considerably greater constitutive concentrations of the transcript in spleen than in liver. Putative splenic CYP2B1 mRNA was significantly elevated by the PB treatment, although not as profoundly as the hepatic response. In contrast, only the livers of the barbiturate-treated rats expressed CYP2B1 proteins or specific catalytic activity (androstenedione 16beta-hydroxylase). Protein and catalytic activities of the isoforms were undetectable in spleen of either male or female vehicle- and PB-treated rats. In agreement, RT-PCR was unable to demonstrate the expression of splenic CYP2B1 mRNAs. Investigating the possibility that the Northern probe for CYP2B1 was identifying a similar sequence isoform, we performed RT-PCR using primers for CYP2B12 and CYP2B15. Since neither of these isoforms was expressed in spleen, we conclude that the spurious results using the Northern probe for CYP2B1 mRNA were due to the presence of a cross-reacting, PB-responsive transcript not currently identifiable in existing databases.

Interactions of gender, growth hormone, and phenobarbital induction on murine Cyp2b expression

The interactions of gender, growth hormone, and phenobarbital induction on Cyp2b expression were examined in phenotypically normal (lit/+) and growth-hormone deficient "little" (lit/lit) mice. Using an immunocrossreactive monoclonal antibody designed to identify rat CYP2B1 and 2B2 proteins, we observed three hepatic Cyp2b proteins in control (lit/+) females, but only two proteins, one at trace levels, in control males. Phenobarbital administration to lit/+ mice increased the expression of the two Cyp2b isoforms in the males by 3- to 4-fold, but produced an approximately 75% reduction in the female-expressed proteins. Whereas growth hormone depletion (lit/lit) had no effect on the expression profile of Cyp2b proteins in females, it had a de-repressive effect in males, resulting in the expression of three proteins at concentrations now comparable to those observed in female liver. Generally, phenobarbital had no inductive effects in the lit/lit mice of both sexes. In all groups, transcript levels measured by a CYP2B1 probe were in agreement with the protein findings. In contrast, Cyp2b mRNA identified by an oligonucleotide probe for CYP2B2 were repressed completely by growth hormone in both sexes, and was expressed as a female-predominant transcript in the lit/lit mice. In spite of an apparent high degree of sequence homology between the rat CYP2B and murine Cyp2b gene families, the present findings highlight fundamental differences in their constitutive and gender-dependent expression, growth hormone regulation, and phenobarbital inducibility.